As I have very little approaching a social life I decided to have a go at making a forecast of where one can keep Beautiful Firetails! It's made using a thing called Spacial Data Analysis, where a program finds all the geocoded records of a species, and records the environment from those spots, and turns that into a forecast of where they're likely to be found!
In this case I decided that it would be best to add only climate layers to read the data from:
Below is the full list
Occurrence Density
Evaporation - annual mean
Evaporation - average
Growth index C3 macrotherm plants - annual mean
Growth index C3 mesotherm plants - annual mean
Growth index C4 megatherm plants - annual mean
Humidity - annual mean relative
Aridity index - annual mean
Precipitation - annual
Precipitation - driest month
Solar Radiation - annual mean
Solar Radiation - highest period
Solar Radiation - lowest period
Temperature - annual max mean
Temperature - annual mean
Temperature - annual min mean
Temperature - coldest month min
Temperature - warmest period max
and the map is here [Link removed, look at the map lower down, more accurate]
Basically, if one wants to keep Beautiful Firetails the optimal climate you can keep them in is
With a total maximum temperature of between 15 and 25 (and if you have a maximum of over 30, it's very difficult)
An annual mean temperature of between 8 and 15 (any higher than 18... Ohh oo)
Over 1000mm of rain a year
With no month having less than 40mm
The birds are not sensitive to a minimum temperature
An average humidity of over 80%
For Matty:

That is a suitability map zoomed and centred around Canberra...
Gundaroo doesn't seem like the best area for them, though I think the major points are the one covered above, so if your place adheres to all the above, you should be fine!
I think it's the Growth indexes that dragging Gundaroo down... So yeah!
Of course, this is wild birds, but I assume that Beautiful Firetails aren't domestic enough for them to be very different from the Wild Birds
~Nathan
Edit: The Values for Gundaroo are as follows:
Annual Max Mean (Temperature): 17.37 (So you're good there)
Annual Mean Temperature: 13.6 (Fine there aswell)
Total Annual Rainfall: 638mm (A little low there, but the firetails might be exclusively in above 1000mm because the plants they live in need more than 1000mm, if you can supply those plants, should all be good, if not, just sprinkle them with the hose abit xD)
Rainfall in the Driest Month: 47mm (All okay there)
Average Humidity: 81%
So I think you should be all right!
Edit 2:
Just did another Analysis without the Growth Indexes, and although Gundaroo hasn't faired much better it's alot more forgiving to places in general
http://imageshack.us/photo/my-images/692/species2.png/" onclick="window.open(this.href);return false;
I think you're (Matty) biggest problem is Solar Radiation, where the Annual Mean Gundaroo is at the very top end of what they can stand, and so is the maximum Solar Radiation, I think a Sun Shade or roofing over most of the aviary would be best for them ;)